1. Field
The present disclosure relates generally to light-emitting diode (LED) bulbs and, more specifically, to an LED bulb that produces shifting color output as the luminous flux of the LED bulb is reduced.
2. Description of Related Art
Traditionally, lighting has been generated using fluorescent and incandescent light bulbs. While both types of light bulbs have been reliably used, each suffers from certain drawbacks. For instance, incandescent bulbs tend to be inefficient, using only 2-3% of their power to produce light, while the remaining 97-98% of their power is lost as heat. Fluorescent bulbs, while more efficient than incandescent bulbs, do not produce the same warm light as that generated by incandescent bulbs. Additionally, there are health and environmental concerns regarding the mercury contained in fluorescent bulbs.
Thus, an alternative light source is desired. One such alternative is a bulb utilizing an LED. An LED comprises a semiconductor junction that emits light due to an electrical current flowing through the junction. Compared to a traditional incandescent bulb, an LED bulb is capable of producing more light using the same amount of power. Additionally, the operational life of an LED bulb is orders of magnitude longer than that of an incandescent bulb, for example, 10,000-100,000 hours as opposed to 1,000-2,000 hours.
Traditional incandescent bulbs are capable of producing variable levels of light output by, for example, reducing the electrical power applied to the filament element. Typically, as an incandescent bulb is dimmed, it produces a warmer or red-shifted light color. Because we are accustomed to incandescent bulbs, when the light output of a bulb is reduced we commonly expect the light color to also be red-shifted to produce a dimmed, warm light output. In some lighting scenarios, such as indoor residential lighting, the red-shifted color may even be a desirable result.
The red-shifting of an incandescent bulb is due, at least in part, to the properties of the filament used to produce the light. Typically, as the light output of an incandescent bulb is reduced (the bulb is dimmed), the filament cools and the black-body color temperature of the emitted light is also reduced. The black-body color temperature (CCT) represents the color of light emitted from an ideal (Planckian) black-body at the specified absolute temperature. A reduction in the black-body color temperature is typically perceived as a red-shift in the color of the emitted light which may be perceived as a “warmer” light (even though the black-body color temperature is actually reduced).
In some applications, LED bulbs may also be dimmed to produce reduced levels of light output. However, in contrast to a traditional incandescent bulb, as the light output of an LED is reduced, the color of the light emitted by the LED remains relatively constant. As a result, the light produced by a traditional LED bulb remains at the same black-body color temperature as the LED bulb is dimmed.
In some cases, it may be desirable to provide an LED bulb that produces a variable light output that approximates the variable light output of a traditional incandescent light bulb. The techniques described herein may be used to achieve a color shift as the light output of the LED bulb is changed.